1. Field of the Invention
The present invention relates to a device for detaching a sheet-shaped copy support from a moving photoconductive layer by use of a pressure medium. In the environment of the invention, a nozzle arrangement is positioned ahead of an image transfer station and near the photoconductive layer. The nozzle arrangement consists of several individual nozzles. Upon the nozzle arrangement, a first pressure can act exclusively or initially for a certain period of time followed subsequently by a second lower pressure of the medium.
2. Description of the Prior Art
A device of this kind is known from German Offenlegungsschrift No. 2,262,693 which discloses a detaching device with which, without the use of a neutralizing corona discharge device, the leading edge of a copy support can be separated by compressed air from the moving photoconductive layer, and in which detaching and feeding of the copy sheet to a transport device are achieved by means of compressed air alone. In this detaching device, a high pressure initially acts upon the nozzle arrangement for a certain period of time to produce a high flow rate, and this is followed by a lower pressure for producing a decreased flow rate. The high pressure is switched on when the leading edge of the sheet approaches the nozzle arrangement for detaching the sheet, and this high pressure is then switched off again when the nozzle arrangement is partially covered by the sheet. Subsequently, the lower pressure is switched on or maintained for guiding the sheet along the desired path very closely past the nozzle arrangement.
In addition to the foregoing type of detaching device operation with compressed air, a device is known for detaching the copy sheet wherein the leading edge of the sheet is so irradiated by means of a neutralizing corona discharge device that it is no longer electrostatically retained on the photoconductor. To detach the copy sheet, air is blown between the photoconductive layer and copy sheet, and the copy sheet is conveyed on this air cushion to another transport device. Because of the high velocity of this air, detaching of the copy sheet and of the photoconductive layer is generally not possible without the unfixed toner image on the copy sheet being destroyed.
With the initially described detaching device, the copy sheet is blown only from the back so that a suction pressure, i.e., a pressure below atmospheric, is obtained due to the Bernoulli principle. The copy sheet is moved by this partial vacuum until it is very close to the nozzles but without closing them.
There is a clearly defined distance between the nozzle outlets and the copy support or copy sheet at which maximum suction is exerted upon the copy support. When the copy support is further moved in the direction of the nozzles, that is, when the distance between the copy support and the nozzle outlets is steadily decreased, the suction effect steadily decreases, becomes zero, and finally changes into a positive pressure exerted upon the copy support, such positive pressure being greater the more the nozzle is covered by the copy support. In practice, this leads to the fact that the copy support undergoes periodic oscillations due to its mass and due to the above- described suction pressure behavior.
A possible explanation therefor may be found in that, when the copy support more closely approaches the nozzle outlets, the exhausting air no longer passes tangentially along the back of the copy support to form a partial vacuum but impinges upon the back of the copy support at a certain acute angle with respect to its surface. Thus, a change of the initial suction pressure to a positive or superatmospheric pressure occurs. The copy support is thereby blown away from the nozzle outlet until its distance therefrom is so great that a partial vacuum according to the Bernoulli principle again developes, and the suction procedure begins again. By this periodic oscillation procedure, an extremely intensive whistling tone of a frequency of about 7,000 to 9,000 cycles per second is produced. Despite optimization of all parameters of the nozzle arrangement, this undersired sound cannot be suppressed. Rather, it is of such an intensity that it hardly can be damped to a normal sound volume, even with the use of extensive sound damping measures.